1. Field of the Invention
The field of the invention is generally that of polyurethane resins. More specifically, it is that of polyurethane resins specifically designed for use in the preparation of magnetic tapes suitable for use in audio, video and computer applications. A magnetic tape is produced essentially by adhering a coating of magnetic iron oxide to a suitable flexible substrate, preferably a polyester film.
The task of the resin employed in magnetic tapes is to hold and bind together the magnetic pigment, which is the fundamental material which permits a recording to be made.
Obviously, the more magnetic iron oxide which can be bonded to the substrate to form the tape the better. Of course, the binding agent must not impart any undesirable qualities to the finished tape.
2. Description of the Prior Art
The prior art is replete with teachings of the preparation of polyurethane resins made by reacting a diisocyanate with a polyester, where the polyester is formed from the reaction of a glycol with a dicarboxylic acid, with or without a chain extender. Thus, some U.S. prior art patents which have been located and which bear on this subject matter are the following: U.S. Pat. Nos. 2,871,218; 2,888,432; 2,983,702; 3,660,341; 3,357,954; 3,718,518; 3,753,953; 3,923,747; 3,929,732; 3,931,113; 4,000,117; 4,010,311; and 4,049,590.
However, none of said prior art patents teaches the preparation of a polyurethane resin which can function as an efficient binder of magnetic iron oxide to a flexible substrate, without the necessity of hardening or crosslinking agents, to produce a magnetic tape which exhibits the requisite properties of hardness, freedom from blocking, good mechanical properties and a high magnetic iron oxide holding power.
Current urethane resin products available to the magnetic tape industry have several disadvantages:
A. They must be compounded with non-urethane resin modifiers such as phenoxides, epoxies and vinyls in order to increase their hardness and reduce their elastomeric nature.
B. They must be crosslinked with polyfunctional isocyanates at substantially high levels (10-15% by weight based on the resin) to produce satisfactory coatings and overcome blocking problems encountered during the processing and storage of the magnetic tape.
The term "blocking" is virtually synonymous with "stickiness" and describes the condition in which a film or tape, when wound up into a roll, cannot be unwound because it has adhered to itself.
C. They do not have maximum magnetic pigment or magnetic iron oxide holding or binding power.
It would, therefore, be desirable to provide polyurethane resins which combine a unique set of properties in which
(1) The mechanical properties have been balanced and the need of compounding with non-urethane modifiers has been eliminated.
(2) Essentially no crosslinking is required to produce totally satisfactory magnetic tape coatings. This is a definite improvement, since the crosslinking reaction is considered by the industry to be the most critical and difficult step to control during the production of magnetic tape.
(3) The blocking characteristics are satisfactory even when the polymer is used uncrosslinked.
(4) Superior magnetic pigment holding power is displayed.